Experimental investigation of the pre-tension effects on the modal parameters of a slender pre-tensioned concrete beam

Slender pre-tensioned concrete structures are nowadays of common use, due to their unique features. Slender beam can be used to sustain large structures and important loads leaving the architects a lot possibilities in the structure design. The health of such structures is strictly related to the conditions of the tendon that, in many cases, are not accessible. Visual inspection is the most used technique to assess the tendon condition even if in many cases access to the inspection points is very difficult or impossible This paper investigates the possibility of detecting a pre-stress loss or a tendon failure by means of modal analysis. A pre-stressed concrete beam has been built and tested under laboratory conditions, varying the applied pre-stress and the number of active tendon. Modal analysis has been performed in each of the experimental condition and attention has been focussed on the variation of all identified modal parameters, frequency, damping and mode shapes. Furthermore a set of damage indexes have been computed to highlight the most sensible magnitude able to identify a change on the structure. Obtained results showed that an accurate analysis is needed in order to identify a change in modal parameters due to variations in the pre-stress, while tendon failure leads to more important changes in the identification results. The performed study are the starting point in order to properly tune a numerical model of the beam useful to accurately interpreter structural changes. ©2010 Society for Experimental Mechanics Inc.